test: call toLowerCase on the resolved module

The commit updates test-require-resolve.js to call toLowerCase on the
resolved module instead of the path. Currently this test will fail if
the path to where node exists contains uppercase letters. For example:

```
$ out/Release/node
test/parallel/test-require-resolve.js
/root/rpmbuild/BUILD/node-v8.8.0/test/parallel
module.js:515
    throw err;
    ^

Error: Cannot find module
'/root/rpmbuild/build/node-v8.8.0/test/fixtures/nested-index/one'
    at Function.Module._resolveFilename (module.js:513:15)
    at Function.resolve (internal/module.js:18:19)
    at Object.<anonymous>
(/root/rpmbuild/BUILD/node-v8.8.0/test/parallel/test-require-resolve.js:37:11)
    at Module._compile (module.js:612:30)
    at Object.Module._extensions..js (module.js:623:10)
    at Module.load (module.js:531:32)
    at tryModuleLoad (module.js:494:12)
    at Function.Module._load (module.js:486:3)
    at Function.Module.runMain (module.js:653:10)
    at startup (bootstrap_node.js:187:16)
```

PR-URL: nodejs/node#16486
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>>
Reviewed-By: Anna Henningsen <anna@addaleax.net>>
Reviewed-By: James M Snell <jasnell@gmail.com>>

test/parallel/test-require-resolve.js

@@ -32,7 +32,7 @@ assert.strictEqual(
   require.resolve(fixtures.path('a')).toLowerCase());
 assert.strictEqual(
   fixtures.path('nested-index', 'one', 'index.js').toLowerCase(),
-  require.resolve(fixtures.path('nested-index', 'one').toLowerCase()));
+  require.resolve(fixtures.path('nested-index', 'one')).toLowerCase());
 assert.strictEqual('path', require.resolve('path'));
 
 // Test configurable resolve() paths.